Phyllodes tumor (cystosarcoma phyllodes) of the breast. A clinicopathologic study of 45 cases.

The correlation between the histology and biologic behavior of phyllodes tumor was studied in order to establish the histologic criteria of this tumor, particularly its malignant counterpart. A preliminary analysis of five tumors with metastases revealed that a single essential indicator of malignant potential was the presence of a disproportionate overgrowth of the stromal elements at the expense of the ductal element. The presence or absence of such stromal overgrowth in the tumor showed a positive correlation with the degree of stromal atypism and the rate of mitoses. Seventeen out of 45 tumors were determined to be malignant, on the basis of our criteria. The arrangement of the stromal component in malignant phyllodes tumors was usually one of the particular sarcomatous types, with or without the focal areas of one of the specified types of sarcoma. On the other hand, 24 tumors recurred locally but never metastasized, and were preferentially interpreted to be benign phyllodes tumor. Histologically, these tumors had a general architecture resembling that of fibroadenoma but were characterized by high cellularity expressed by the distinct formation of bundles or fascicles of stromal tumor cells. The remaining four tumors seemed to form a borderline phyllodes tumor subset. A follow-up study revealed that, irrespective of the mode of therapy, the outcome of patients with phyllodes tumor was well correlated with the histologic criteria we proposed.